1. Field
The present invention relates generally to communication, and more specifically to techniques for calibrating downlink and uplink channel responses in a wireless multiple-input multiple-output (MIMO) communication system.
2. Background
A MIMO system employs multiple (NT) transmit antennas and multiple (NR) receive antennas for data transmission. A MIMO channel formed by the NT transmit antennas and NR receive antennas may be decomposed into NS spatial channels, where NS≦min {NT, NR}. The NS spatial channels may be used to transmit data in parallel to achieve higher overall throughput or redundantly to achieve greater reliability.
To achieve high performance, it is often necessary to know the response of the MIMO channel. For example, an access point may need to know the response of a downlink channel in order to perform spatial processing for a downlink transmission to a user terminal. In one conventional channel estimation technique, the access point transmits a pilot on the downlink, and the user terminal estimates the downlink channel response based on the pilot and sends the downlink channel response estimate back to the access point. This channel estimation technique utilizes uplink resources and further incurs a delay to send back the channel response estimate, both of which are undesirable.
A TDD system uses a single frequency band for both the downlink and uplink, with the downlink being allocated a portion of the time and the uplink being allocated the remaining portion of the time. For a TDD system, the downlink and uplink channel responses may be assumed to be reciprocal of one another. That is, if H represents a channel response matrix from antenna array A to antenna array B, then a reciprocal channel implies that the coupling from array B to array A is given by HT where HT denotes the transpose of H. With a reciprocal channel, the channel response for one link (e.g., the downlink) may be estimated based on a pilot received via the other link (e.g., the uplink).
The access point and user terminal both utilize transmit and receive chains for transmission and reception, respectively. A downlink transmission would then observe an “effective” downlink channel response that includes the responses of the transmit chain at the access point and the receive chain at the user terminal. Correspondingly, an uplink transmission would observe an effective uplink channel response that includes the responses of the transmit chain at the user terminal and the receive chain at the access point. The responses of the transmit and receive chains at the access point are typically different from the responses of the transmit and receive chains at the user terminal. As a result, the effective downlink and uplink channel responses are typically not reciprocal of one another. If the channel response estimate obtained for one link is used for spatial processing for the other link, then any difference in the responses of the transmit/receive chains at the access point and user terminal would represent error that, if not determined and accounted for, may degrade performance.
There is, therefore, a need in the art for techniques to calibrate the downlink and uplink channel responses in a TDD MIMO system.